Protective coating

ABSTRACT

A method of producing a coating on a substrate by electrolytically co-depositing a metal matrix M 1  and particles of CrAlM 2 , where M 1  is Ni, Co or Fe or two or all of these elements and M 2  is Y, Si, Ti, Hf, Ga, Nb, Mn, Pt, a rare earth element or two or more of these elements. The co-deposition is carried out at a current density of less than 5mA per square centimeter. Preferably, the co-deposition forms a layer less than 50 microns thick, and occurs at a bath loading of less than 40 grams per liter of the particles. In a preferred embodiment, the particle size distribution in the plating bath is 25 percent between 15 and 12 microns, 45 percent between 12 and 10 microns and 30 percent less than 10 microns. The method is particularly useful for coating a gas turbine part.

This is a national stage application of PCT/GB95/01746 filed Jul. 24,1995.

The present invention relates to the provision of protective coatings,such as overlay coatings, on substrates. Such coatings are employed oncomponents which are subjected to high temperature environments,particularly where corrosion and/or erosion is likely to occur. Theprimary but not necessarily sole application of such coatings is toparts of gas turbine engines, particularly superalloy componentsthereof, such as gas turbine shafts, rings, disks, combustion can ware,stator and rotor blades and guide vanes. The invention also relates tosuch parts, and to machinery and vehicles or fixed installations whichincorporate such parts.

It has long been recognised that components of gas turbines,particularly the internal components thereof in the vicinity of anddownstream of the combustor, need to exhibit high strength and corrosionresistance at high temperature.

It is known to provide such components with a load bearing structure ofsuperalloy material, to provide sufficient high temperature strength.Typical superalloys used (examples are those known under the tradedesignations IN100, IN718, IN738, MAR-M002, MAR-M247, CMSX-4, PWA1480and PWA1484) are the Ni, Co and Fe base superalloys, dependent upon theparticular application requirements. Fe and Co base superalloys areoften solid solution strengthened. Ni base alloys have Ni as the majorconstituent and often contain amounts of Cr Co Fe Mo W or Ta, and areoften solid solution or precipitation strengthened. Precipitationstrengthened Ni base alloys are widely used for gas turbine componentsand often contain Al Ti or Nb to produce a precipitated second phase inthe course of a suitable heat treatment. Examples of Ni baseprecipitation strengthened superalloys used for gas turbine componentsare those known under the trade designations INCO 713, B-1900, IN 100,MAR-M 200, and MAR-M 247. Examples of Co base superalloys are MAR-M 509and Haynes 188, and examples of Fe base superalloys are Incoloy 802 andIncoloy 903. Superalloy gas turbine components are sometimes wrought orcast and, for the more extreme operating conditions, may bedirectionally solidified or in the form of single crystal structures.

It has become common practice to coat superalloy components withcorrosion resistant material since the superalloy itself will notnormally be capable of withstanding the corrosive/oxidative in-serviceatmosphere.

One practice is to aluminise the superalloy. This is usuallyaccomplished using a so-called pack aluminising process, or by physicalvapour deposition. These processes involve diffusion of Al into thesuperalloy to form aluminides such as NiAl in the case of Ni basesuperalloys. In service, a surface layer of Al₂ O₃ forms to protect thematerial beneath and this tends to exfoliate due to thermal expansionand contraction. This is gradually repaired by outwardly diffusing Aland finally, when there is no longer sufficient Al to replace exfoliatedmaterial at a particular location, the superalloy component will beliable to rapid localised corrosion. Chromium and silicon eithertogether or singly and alone or in addition to aluminium may likewise bediffused into the superalloy to form a surface layer including chromidesor silicides. Although reference will be made hereafter mainly toaluminising it should be understood that such reference should beinterpreted as alternatively referring mutatismutandis to chromisingand/or siliconising.

A further practice is to coat the superalloy with an overlay of, forexample, MCrAlY, MCrAlHf, MCrAlYHf, MCrAlYHfSi and MCrAlTaY where M isCo or Ni or Fe or a mixture thereof. The addition of Y Si or Hf helps toprevent exfoliation of Al₂ O₃ from the surface and thus extends the lifeof the component. These materials may be applied by plasma spraying; orby a co-deposition process, such as the process we describe in ourpatent GB-B-2 167 446. It is usual to coat a component with thesematerials to produce a layer 75 to 200 μm or more in thickness. Thecoating processes are expensive and coating components to this order ofthickness gives them a life long enough to justify the cost.

A problem with layers of this order of thickness is that they tend to besusceptible, as in-service conditions become gradually more extreme inmore modern gas turbines, to thermal mechanical fatigue cracking andthis is highly undesirable, particularly if the coating is applied to athin-walled hollow superalloy component such as a turbine blade, sincethe cracking of the coating can cause blade failure.

U.S. Pat. No. 4 897 315 describes plasma spraying of a 0.001 inch (25.4μm) layer of NiCoCrAlY on to single crystal Ni base superalloy. Afterplasma deposition, the coating is glass bead peened, aluminised with apack cementation mixture, and is, finally, put through diffusion andprecipitation heat treatment steps. The specification indicates that thepreferred method of applying the MCrAlY coating is by plasma spraying,but the specification also makes a general statement that the MCrAlY canbe applied by, e.g., plasma spraying, electron beam evaporation,electroplating, sputtering, or slurry deposition. One reason for thepeening operation in the prior art is thought to be that theas-deposited MCrAlY is not particularly smooth.

Although U.S. Pat. No. 4 897 315 mentions other ways to deposit theMCrAlY, no particular process is known which is capable of producing acoating which has both excellent corrosion resistance and excellentresilience to thermal cycling. It is believed that any known platingmethod either produces a coating which is too porous with poor thicknesscontrol on some sections, or is susceptible to cracking under thermalcycling.

In our patent GB-B-2 254 338, we disclose a method of co-depositingCoCrAlY with a current density of 3 mA per square centimetre and bathpowder concentration of 70 g/l for a period of 24 hours to produce acoating thickness of between 50 and 125 μm. Although some of the platingparameters described in GB-B-2 254 338 are of similar magnitude to thoseused in the example set out below to illustrate the present invention,the differences are believed to have significant effects on thestructure of the coating produced in practice. GB-B-2 254 338 isdirected specifically towards an apparatus and process for overcomingthe problems of coating complex or abruptly changing shapes, and is notdirected towards solving the same problems as those the presentinvention sets out to solve.

The present invention aims to alleviate the problems of the prior artand, in particular, aims to provide a process capable of producing acoating which is non-porous, smooth with good thickness control on allsections, and which is not easily susceptible to cracking under thermalcycling.

According to a first aspect of the present invention there is provided amethod of producing a coating on a substrate which comprises depositingby electrolytic deposition a metal matrix M₁ from a bath containingparticles of CrAlM₂ so as to co-deposit the particles with the matrix,M₁ being Ni or Co or Fe or two or all of these elements and M₂ being Y,Si, Ti, Hf, Ta, Nb, Mn, Pt, a rare earth element or two or more of theseelements, the deposition being carried out at a current density of lessthan 3 mA per square centimetre.

According to a second aspect of the present invention there is provideda method of producing a coating on a substrate which comprisesdepositing by electrolytic deposition a metal matrix M₁ from a bathcontaining particles of CrAlM₂ so as to co-deposit the particles withthe matrix in the form of a layer, M₁ being Ni or Co or Fe or two or allof these elements and M₂ being Y, Si, Ti, Hf, Ta, Nb, Mn, Pt, a rareearth element or two or more of these elements, the deposition beingcarried out at a current density of less than 5 mA per squarecentimetre, the layer being less than 50 μm thick.

According to a third aspect of the present invention there is provided amethod of producing a coating on a substrate which comprises depositingby electrolytic deposition a metal matrix M₁ from a bath containingparticles of CrAlM₂ so as to co-deposit the particles with the matrix,M₁ being Ni or Co or Fe or two or all of these elements and M₂ being Y,Si, Ti, Hf, Ta, Nb, Mn, Pt, a rare earth element or two or more of theseelements, the deposition being carried out at a current density of lessthan 5 mA per square centimetre and at a bath loading of less than 40g/l of the particles.

In each above aspect, where not mentioned as a feature of the aspect, itis preferable for the process to be carried out at a current density ofless than 3 mA per square centimetre. Likewise, it is preferable for thematrix material and particles to be co-deposited in the form of a layerwith thickness less than 50 μm; likewise, it is preferable for thedeposition to be carried out at a bath loading of less than 50 g/l ofthe particles.

In this invention, we have found that a relatively low current densityis a particularly important parameter in the coating process. Previousto the present invention the current density had not been thoughtparticularly important in most plating processes and, where it has beenconsidered important, it has been considered desirable to plate at asignificantly higher current density; see, for example, U.S. Pat. No. 5064 510 where a current density of 50 to 80 mA per square centimetre isused and is said to achieve advantageously a high deposition rate withinthe range of 100 μm/h to 150 μm/h.

It is considered that there is a prejudice in the art against using arelatively low current density; the reason for this is not known exactlybut one explanation may be that the coating process is slower at lowcurrent densities thus adversely affecting the overall cost.

In this invention, we prefer a current density of less than 2.5 mA persquare centimetre. A current density of less than about 2 mA per squarecentimetre is even more preferable in some circumstances, a currentdensity of about 1 mA per square centimetre being one example.

At the relatively low current densities employed in this invention, wenote a tendency for the constitution of particles on the as depositedcoating to differ from the constitution in the bath, in that smallerparticles are preferentially incorporated (eg using <15μm powder,the >10 μm particles are not incorporated so preferentially as the <10μm particles). This is particularly surprising since, in theory basedupon Faraday's Law and Stokes' equation (see Transactions of theInstitute of Metal Finishing, article entitled: "The Production ofMulti-Component Alloy Coatings by Particle CoDeposition," by J. Fosteret al, pp. 115-119, Vol. 63, No. 3-4, 1985) and assuming suitableconditions of current density and agitation are used, the larger theparticle size the smaller the bath loading needs to be to achieve aparticular fraction of powder incorporated in the as deposited coating.One would therefore expect larger particles to be preferentially plated,but we have found that at relatively low current densities, the oppositeoccurs. This phenomenon which occurs in practice is thought to be atleast partly responsible for the excellent coatings which are achievablein the present invention and which overcome the problems of the priorart.

In one embodiment, the M₁ comprises Co. This helps to promote aparticularly smooth coating. If it is desirable for Ni to be present inthe coating, a flash of Ni may be electroplated either on top of theco-deposited material, or directly onto the substrate before theco-deposition step. The flash of Ni may be about 5 μm thick.

It is preferred for the metal matrix material and particles to beco-deposited to form a layer less than 25 μm thick. In a particularlypreferred embodiment, the layer may be about 15 μm thick. However, thelayer may be less than 15 μm thick, about 12 or 10 μm (or less thanthese values) being examples. For most applications, it is preferablefor the layer to be more than or equal to 5 μm thick, more preferablystill for it to be more than or equal to 10 μm thick. However, the layermay, for some applications, be more than 15 μm thick.

As mentioned above, it is preferable for the deposition to be carriedout at a bath loading of less than 40 g/l of the particles. Morepreferably, a bath loading of about 30 g/l, or less than 30 g/l, isused. More preferably still, a loading of about 20 g/l, or less than 20g/l, is used. In a particularly preferred embodiment, a bath loading ofabout 10 g/l is used, although lower loadings, such as about 1 g/l, areenvisaged. These relatively low bath loadings ensure that the depositedcoating does not build up in a porous manner and is not rough.

The particles may be spherical, and may be formed using an atomiser,such as a nozzle atomiser. Preferably the particles in the bath comprise<15 μm <12 μm, or <10 μm powder.

In one preferred embodiment, the particle distribution in the bathconsists of 25% between 15 and 12 μm, 45% between 12 and 10 μm, and 30%less than 10 μm. We have found, surprisingly and unexpectedly, thatplating at relatively low current densities results in small particlesbeing preferentially deposited onto the substrate; when powder with thisin-bath distribution is used, a distribution in the as-deposited coating(as a weight percentage of the amount of powder in the deposit) of 45%<10 μm, 55% between 10 and 12 μm, and 0% between 12 and 15 μm mayresult.

Excellent coatings are achievable with processes incorporating thisrefinement step and, preferably, a refinement step is included in theco-deposition step.

In one embodiment, a layer of protective material is co-deposited whichcomprises only a mono-layer or duo-layer of particles. For example, whena powder having a particle size less than 15 μm is suspended in thebath, due to the refinement, it is possible to put down, as desired, asubstantially continuous 12 or 10 μm mono-layer of particles (thelargest as-deposited particles being 12 or 10 μm in size respectively)although in practice, it is unlikely that for any given desiredthickness of coating one would employ powders having particles of a sizegreater than this desired thickness. In another preferred process, 4 to8 μm powder may be used to provide a duo-layer or trio-layersubstantially 10, 12, 15 or 20 μm thick, as desired.

In one embodiment, the substrate onto which co-deposited material isapplied comprises a superalloy component which may comprise a componentof a gas turbine. In another embodiment, the co-deposited material maybe applied on top of a Ni flash (eg 2 μm thick) plated on the surface ofthe superalloy component. After co-deposition, the co-deposited materialmay consist of more than 40% (by volume) of the particles, and in someapplications, 45% may be exceeded.

During the co-deposition process, gas, such as air or an inert gas, maybe admitted to the bath at a location to produce circulation in thesolution generally upwards in one zone and generally downwards in asecond zone, the substrate being located in the second zone duringco-deposition. The substrate (or component of which it forms part) maybe rotated about an axis which is horizontal or has a horizontalcomponent during co-deposition. Electrodeposition apparatus as describedin our patent GB-B-2182055 may be used.

In some circumstances, it may be desirable to rotate the substrate abouta first axis having a horizontal component, and to rotate the substrateabout a second axis which is non-parallel with the first. The cycle ofrotation about the first axis may include periods of higher angularvelocity and periods of lower angular velocity. The second axis may beperpendicular to and/or intersect the first axis. The cycle of rotationabout the first axis may be alternately stop and go. When the substrateis only rotated about one axis having a horizontal component, therotational cycle may include periods of higher angular velocity andperiods of lower angular velocity, and rotation may also be alternatelystop and go. Manipulation of the substrate may be in accordance with theprocess described in our patent GB-B-2221921.

In a most preferred embodiment, the co-deposited material issubsequently aluminised, for example by pack or vapour phasealuminising. A pre-diffusion heat treatment may be included between theco-deposition and the aluminising. Following the aluminising step, apost-diffusion heat treatment may be employed, preferably followed by anage hardening step.

A platinum deposition step may be included before or after aluminising,preferably before. The platinum deposition preferably comprises platinga layer of platinum (in the region of 5 to 10 μm thick) on top of theco-deposited material. The pre-diffusion step preferably comprisessubjecting the co-deposited material (and, optionally, the platinumlayer if there is one) to between 1000° and 1100° C. for approximatelyone hour in vacuum. When a pack aluminising process is used, thispreferably occurs at approximately 900° C. for approximately 6 hours.The post-diffusion step preferably comprises subjecting the aluminizedcoating to approximately 1100° C. for approximately one hour in avacuum. The age hardening step preferably comprises subjecting thepost-diffused coating to approximately 870° C. for approximately 16hours in a vacuum.

One preferred process involves the application of a substantially 15 μmthick layer of co-deposited material to the substrate, followed byaluminising or platinum aluminising, and heat treatment. After heattreatment, the total thickness of coating material on the superalloy ispreferably less than 75 μm.

The substrate may comprise any gas-washed substrate of a gas turbinecomponent, such as the aerofoil, root or shroud portions of a blade.

In addition to or as an alternative to aluminising or platinumaluminising the co-deposited material as discussed above, a thermalbarrier layer, for example of columnar material, may be deposited as afinal layer. The thermal barrier may comprise a ceramic material, suchas yttria-stabilised zirconia.

According to a fourth aspect of the present invention there is provideda method of manufacturing or overhauling a gas turbine component whichincludes coating a substrate of the component according to any one ormore of the first second and third aspects of the invention.

According to a fifth aspect of the present invention there is provided agas turbine component or a gas turbine including a componentmanufactured or overhauled according to the fourth aspect of theinvention.

According to a sixth aspect of the present invention there is provided avehicle or a fixed installation including a gas turbine according to thefifth aspect of the invention. A vehicle according to this aspect of theinvention may comprise, for example, an aircraft, or a water or landvehicle.

The invention may be performed in various ways but one method of coatingwill now be described by way of example with reference to theaccompanying diagrammatic drawings, in which:

FIG. 1 is a perspective view of a coating apparatus;

FIG. 2 is a side elevation of the apparatus;

FIG. 3 is a front elevation of the apparatus; and

FIG. 4 is a perspective view of a jig on which the articles to be platedare suspended.

The apparatus shown in the drawings, comprises a vessel or container 1having a parallelepiped shaped upper portion 2 and a downwardly taperinglower portion 3 in the form of an inverted pyramid which is skewed sothat one side face 4 forms a continuation of one side face 5 of theupper portion.

The vessel 1 contains a partition 6 which lies in a vertical planeparallel to the side faces 4 and 5 of the vessel and makes contact atits side edges 7 and 8 with the adjacent vertical and sloping faces ofthe vessel. The partition thus divides the vessel into a larger workingzone 9 and a smaller return zone 11. At its bottom, the partition 6terminates at a horizontal edge 12 above the bottom of the vessel toafford an interconnection 13 between the working zone 9 and the returnzone 11. At its top, the partition 6 terminates at a horizontal edge 14below the top edges of the vessel 1.

At the bottom of the return zone 11 there is an air inlet 15 which isconnected to an air pump (not shown) Mounted in the working zone 9 is ajig 21 to which the workpieces to be coated are mounted, the jig 21being arranged to move the workpieces within the vessel in a manner tobe described in greater detail below.

When the apparatus is to be used for electrolytic plating, conductorsare provided to apply a voltage to the workpiece mounted on the jig 21relative to an anode which is suspended in the working zone.

To use the apparatus, to co-deposit a coating on the workpieces, theworkpieces are mounted on the jig 21 which is positioned in the vesselas shown. Before or after the positioning of the jig, the vessel isfilled to a level 17 above the top edge 14 of the partition 6 with aplating solution containing particles to be co-deposited. Air isadmitted to the inlet 15 and this rises up the return zone 11, raisingsolution and entrained particles. At the top of the return zone, the airescapes and the solution and particles flow over the broad crested weirformed by the top edge 14 of the partition and flow down past theworkpieces on the jig 21. At the bottom of the working zone 9, theparticles tend to settle and slide down the inclined sides of the vesseltowards the interconnection 13 where they are again entrained in thesolution and carried round again.

As the downwardly travelling particles in the working zone 9 encounterthe workpiece, they tend to settle on the workpiece where they becomeembedded in the metal which is being simultaneously plated out.

As shown in FIG. 4 and as described in GB-B-2 254 338, the workpieces tobe coated are mounted on a jig 21 shown in FIG. 4 which is suspended inthe vessel 1. The jig is shown in simplified form in FIGS. 2 and 3 butomitted from FIG. 1 for reasons of clarity. The jig 21 comprises a deck22 which fits over the top of the vessel 1, a depending pillar 23towards one end and a pair of depending guides 24 at the other end. Theguides 24 have facing guideways in which slides a cross-head 25 carryinga vertical rack 26 which passes upwards through a hole 27 in the deck 22and meshes with a pinion 28 driven by a reversible electric motor 29.The deck 22 supports a second electric motor 31 which drives a verticalshaft 32 carrying a bevel pinion 33 which engages a crown-wheel 34 fixedto one end of a spindle 35 mounted in the pillar 23. The other end ofthe spindle 35 is connected by a universal joint 36 to one end of ashaft 37 the other end of which is carried by a spherical bearing 38 inthe cross-head 25.

The shaft 37 carries a plurality of spurs which are rigidly attachedthereto, only one spur 39 being shown in FIG. 4. The spur 39 extends ina plane containing the axis of the shaft 37 with the longitudinal axisof the spur making an angle α with the axis of the shaft 37. Mounted onthe spur 39 and spaced therealong are three gas turbine blades 42 to becoated, with the longitudinal axes of the blades extending in the saidplane and perpendicular to the longitudinal axis of the spur 39 so thatthe longitudinal axes of the blades make angles of (90-α)° to the axisof the shaft 37.

An electronic motor controller 43 is mounted on the deck 22 and isconnected by lines 44 and 45 to the motors 29 and 31. The controller 43is designed to drive the motor 31 in one direction only but with a stopso as to rotate the shaft 37 about a nominally horizontal axis (thex-axis). The controller 43 is designed to drive the motor 29 alternatelyin opposite directions to reciprocate the cross-head 25 and sosuperimpose on the rotation about the x-axis an oscillatory rotationabout a rotating axis in the universal joint 36 (the y-axis).

The angle α and the parameters of the cycles executed by the motors 29and 31 are selected to suit the workpiece being coated so as to ensurethat all surfaces to be coated spend sufficient time facing generallyupwardly to receive an adequate loading of descending particles to beincorporated in the plated metal as it is deposited. One particularexample of a coating and the method of production thereof will now bedescribed by way of example.

EXAMPLE

The coating is to be produced on a gas turbine blade 42 having anaerofoil section 43 with a root portion 44 at one end and a shroudportion 45 at the other end, the platforms of the root and shroud bothextending at angles of approximately 70° to the axis of the aerofoilportion and the root portion and the shroud portion having end faceswhich extend at respectively 30° and 40° to the circumference of thering of which the blade forms part. For blades of this geometry theangle α is 70°.

It is intended to produce on the aerofoil and platform portions of theblade a coating containing 18.32 weight percent Cr, 8.25 weight percentAl, 0.457 weight percent Y and the remainder cobalt. To produce such acoating the bath is filled with a cobalt plating solution comprising 400grams per litre of CoSO₄. 7H₂ O, 15 grams per litre of NaCl and 20 gramsper litre of boric acid H₃ BO₃.

The bath is maintained at a pH of 4.5 and a temperature of 45° C. Thebath is loaded with powder to a concentration of 10 grams per litre, thepowder having a size distribution of 5 to 12 micrometres and beingcomposed of 67.8 weight percent chromium, 30.1 weight percent aluminiumand 1.7 weight percent yttrium. The size distribution of the powder inpercentages is as follows: 0-2 μm 0.01, 2-4 μm 0.05, 4-6 μm 0.13, 6.8 μm4.43, 8-10 μm 43.61, 10-12 μm 51.77.

Prior to coating the parts of the root and shroud portions which are notbe plated are given a wax mask and the remaining surfaces are given theconventional preparation treatments appropriate to cobalt plating.

The blade is fixed to a jig 50 with its axis (see FIG. 4) at 20° to thex axis of the jig which is horizontal. During plating the x axis of thejig is oscillated plus and minus 25° about the y axis which isperpendicular to 20 the x axis with a cycle time of 3 minutes.

Simultaneously, the jig is rotated about the x axis unidirectionally andthrough 360° with a cycle time of 10 minutes for a complete revolution.However the rotation about the x axis is intermittent with 10 secondstop periods being interspersed with 3 second go periods.

Plating is carried out with a current density of 1.5 amps per squarecentimetre for a period sufficient to produce a coating thickness ofsubstantially 12 microns.

A coating of excellent qualities is produced covering the aerofoilportion and the root and shroud platforms and having a weight fractionof incorporated powder of 0.27.

The smaller particles are plated preferentially and substantially noneof the as-deposited particles are >12 μm in size, the larger particlesremaining in the plating solution (ie those between 12 and 15 μm). Afterremoval of the coated blades from the jig, the masking is removed.

The coated surfaces may then be platinum aluminized by electro platingthereon a 10 μm deposit of platinum, pre-diffusing at between 1000° and1100° C. for one hour in vacuum, pack aluminising at 900° C. for 6hours, post-diffusing at 1100° C. for 1 hour in vacuum and age hardeningat 870° C. for 16 hours in vacuum. The pack aluminising step maycomprise a process with cyclically varying pressure, such as isdescribed in EP-A-0024802.

Palladium or ruthenium could be used instead of or as well as platinum.

Particularly preferred M₂ elements are Y, Hf, and Si.

Coatings produced in accordance with the invention have good oxidationresistance and thermal fatigue resistance.

I claim:
 1. A method of producing a coating on a substrate whichcomprises depositing by electrolytic deposition a metal matrix M₁ from abath containing particles of CrAlM₂, said bath having a particledistribution of 25 percent between 15 and 12 microns, 45 percent between12 and 10 microns and 30 percent of less than 10 microns, to co-depositthe particles with the matrix in the form of a layer wherein M₁ is atleast one element selected from the group consisting of Ni, Co, and Feand M₂ is at least one element selected from the group consisting of Y,Si, Ti, Hf, Ta Nb, Mn, Pt and rare earth elements, said deposition beingcarried out at a current density of less than 5 mA per squarecentimeter, the layer being less than 50 microns thick.
 2. The method ofclaim 1 in which either a single or double layer of particles isdeposited during co-deposition of the metal matrix and particles.
 3. Themethod of claim 1 which further includes aluminising, chromising orsiliconising the co-deposited layer.
 4. The method of claim 3 whichfurther includes plating a 5 to 10 micron thick layer of platinum on topof the co-deposited layer.
 5. The method of claim 1 which comprisesco-depositing a substantially 15 micron thick layer of said metal matrixand said particles on to the substrate, aluminising, chromising orsiliconising the co-deposited layer, and heat treatment; wherein if saidheat treatment occurs before said aluminising, chromising orsiliconising, said treatment comprises subjecting the co-deposited layerto between 1000° C. and 1100° C. for approximately one hour undervacuum, and wherein if said heat treatment occurs after saidaluminising, chromising or siliconising, said treatment comprisessubjecting the co-deposited layer to about 1100° C. for approximatelyone hour under vacuum conditions, the thickness of coating material onthe substrate after said heat treatment being less than 75 micron. 6.The method of claim 1 which further includes depositing a thermalbarrier layer subsequent to said co-deposition.
 7. The method of claim1, wherein the substrate is a gas turbine part selected from the groupconsisting of an aerofoil, root, shroud, turbine shaft, ring, disc,combustion can ware, stator blade, rotor blade and guide vane.
 8. Themethod of claim 1 wherein said deposition is carried out at a bathloading of less than 40 g/l of the particles.
 9. The method of claim 1wherein the layer is less than 25 micron thick.
 10. A method ofproducing a coating on a substrate to be subjected to a high temperatureenvironment, which comprises depositing by electrolytic deposition ametal matrix M₁ from a bath containing particles of CrAlM₂, said bathhaving a particle distribution of 25 percent between 15 and 12 microns,45 percent between 12 and 10 microns and 30 percent of less than 10microns, to co-deposit the particles with the matrix in the form of alayer wherein M₁ is at least one element selected from the groupconsisting of Ni, Co, and Fe and M₂ is at least one element selectedfrom the group consisting of Y, Si, Ti, Hf, Ta, Nb, Mn, Pt and rareearth elements, said deposition being carried out at a current densityof less than 5 mA per square centimeter and at a bath loading of lessthan 40 g/l of the particles, the layer being less than 25 micronsthick.